1. Field of the Invention
The invention relates to a diffractive optical element having a support and a plurality of diffraction structures which are applied thereon and are binary blazed by being split into substructures so that the aspect ratio of the substructures varies locally within an individual diffraction structure.
2. Description of Related Art
Diffractive optical elements with locally varying grating constants have found many applications in optics. Diffractive optical elements are used, for example, to generate wavefront profiles which cannot—or only with great difficulty—be achieved by refractive optical elements such as lenses. Fresnel lenses which make it possible to achieve extremely short focal lengths are also widely used. The use of diffractive optical elements to correct chromatic aberrations in optical systems, which are caused by the dispersive properties of the conventional lens materials with broadband light sources, has been proposed, for example, in EP 0 965 864 A2. It is also feasible to use diffractive optical elements for focusing, collimation and beam splitting of laser light in integrated optics, since diffractive optical elements can likewise be produced photolithographically.
Diffractive optical elements whose diffraction structures are blazed, i.e. having a ramp-shaped profile or a profile approximating a ramp shape, are particularly widespread. In comparison with diffractive optical elements whose diffraction structures have a rectangular profile, higher diffraction efficiencies for a desired diffraction order can be achieved by such blazed diffraction structures. The diffraction efficiency of a diffractive optical element means the fraction of the light incident on the diffractive optical element which contributes to a particular diffraction order.
An article by P. Lalanne et al. entitled “Design and fabrication of blazed binary diffractive elements with sampling periods smaller than the structural cutoff”, J. Opt. Soc. Am. A, Vol. 16, No 5, pages 1143 to 1156, discloses diffractive optical elements having diffraction structures which are blazed by splitting them into substructures. The substructures are designed as bars or pillars whose characteristic dimensions are smaller than the wavelength for which the diffractive optical element is intended. Beyond the zeroth order, the substructures then generate no further diffraction orders which could take up energy. Diffraction efficiencies of 80% or more are therefore possible. Via the dimensions of the substructures and their mutual spacing, a refractive index profile which approximates that of a conventional blazed diffraction structure can be generated on the surface.
If the spacings of the pillars or bars are less than a so-called structure period, then the diffractive substructures can be regarded as homogeneous medium in which only one mode can propagate. Even higher diffraction efficiencies are possible in this case. The value of the structure period depends inter alia on the angle of incidence of the light and the geometry of the substructures.
Besides the high diffraction efficiencies, binary substructures furthermore have the advantage of a large angle acceptance. This means that the high diffraction efficiencies can be achieved even with significant deviations from the ideal blaze angle. Furthermore, the high diffraction efficiencies are achieved with a lower polarization dependency and in a larger wavelength range than with conventional blaze structures.
A disadvantage with diffractive optical elements having blazed diffraction structures, however, is that the bar-shaped or pillar-shaped substructures have to be extremely narrow in the regions inside a diffraction structure where the effective refractive index is intended to be particularly small. This leads to very high aspect ratios. This term denotes the ratio of structure height to structure width of the substructures. Structures having such high aspect ratios entail significant problems in production technology, since it is not possible to generate arbitrarily narrow structures by the conventional production method based on deep etching. For production reasons, therefore, the narrowest substructures have previously been avoided, as is described in the aforementioned article by P. Lalanne et al. Avoiding the smallest substructures, however, reduces the maximum achievable efficiency of the diffractive optical element.